In recent years, portable terminal devices such as mobile phones have been equipped increasingly with various advanced features such as a mail function, a camera function, and also a television function. With such an increased number of functions, the portable terminal devices have required more input operations.
However, the number of operation keys is restricted because of space limitations. Accordingly, a user needs to press keys many times even for execution of one function. Therefore, there are concerns about operability deterioration on this kind of portable terminal devices.
Operability deterioration can be suppressed by disposing an input means other than operation keys. For example, an acceleration sensor may be used as another input means. In this case, the acceleration sensor is built into the main body of the portable terminal device. When the acceleration sensor detects any movement of the portable terminal device's main body, a predetermined command is issued in accordance with the movement.
For example, when a user moves the portable terminal device to draw a character for authentication in the air, the drawn character is recognized based on changes in acceleration. Then, if a result of the recognition matches a pre-registered character, the portable terminal device executes a function such as cancellation of a security lock mode (for disabling key operations).